The keen vision of NASA's Hubble Space Telescope has detected a mysterious gap in a vast protoplanetary disk of gas and dust swirling around the nearby star TW Hydrae, located 176 light-years away in the constellation Hydra (the Sea Serpent). The gap's presence is best explained as due to the effects of a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow.
Researchers, led by John Debes of the Space Telescope Science Institute in Baltimore, Md., found the gap about 7.5 billion miles from the red dwarf star. If the putative planet orbited in our solar system, it would be roughly twice Pluto's distance from the Sun.
The suspected planet's wide orbit means that it is moving slowly around its host star. Finding the suspected planet in this orbit challenges current planet formation theories. The conventional planet-making recipe proposes that planets form over tens of millions of years from the slow but persistent buildup of dust, rocks, and gas as a budding planet picks up material from the surrounding disk. TW Hydrae, however, is only 8 million years old. There has not been enough time for a planet to grow through the slow accumulation of smaller debris. In fact, a planet at 7.5 billion miles from its star would take more than 200 times longer to form than Jupiter did at its distance from the Sun because of its much slower orbital speed and a deficiency of material in the disk.
An alternative planet-formation theory suggests that a piece of the disk becomes gravitationally unstable and collapses on itself. In this scenario, a planet could form more quickly, in just a few thousand years.
"If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties," Debes says. "That might add to planet formation theories as to how you can actually form a planet very far out. There's definitely a gap structure. We think it's probably a planet given the fact that the gap is sharp and circular."
What complicates the story is that the red dwarf star is only 55 percent the mass of our Sun. "It's so intriguing to see a system like this," Debes says. "This is the lowest-mass star for which we've observed a gap so far out."
The disk also lacks large dust grains in its outer regions. Observations from ALMA (the Atacama Large Millimeter Array) show that millimeter-sized (tenths-of-an-inch-sized) dust, roughly the size of a grain of sand, cuts off sharply at about 5.5 billion miles from the star, just short of the gap. The disk is 41 billion miles across.
"Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet-formation models," Debes says.
The Hubble observations reveal that the gap, which is 1.9 billion miles wide, is not completely cleared out. The team suggests that if a planet exists, it is in the process of forming and not very massive. Based on the evidence, team member Hannah Jang-Condell at the University of Wyoming in Laramie estimates that the putative planet is 6 to 28 times more massive than Earth. Within this range lies a class of planets called super-Earths and ice giants. Such a small planet mass is also a challenge to direct-collapse planet-formation theories, which predict that clumps of material one to two times more massive than Jupiter can collapse to form a planet.
TW Hydrae has been a popular target with astronomers. The system is one of the closest examples of a face-on disk, giving astronomers an overhead view of the star's environment. Debes's team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to observe the star in near-infrared light. The team then re-analyzed archival Hubble data, using more NICMOS images as well as optical and spectroscopic observations from the Space Telescope Imaging Spectrograph (STIS). Armed with these observations, they composed the most comprehensive view of the system in scattered light over many wavelengths.
When Debes accounted for the rate at which the disk dims from reflected starlight, the gap was highlighted. It was a feature that two previous Hubble studies had suspected but could not definitively confirm. These earlier observations noted an uneven brightness in the disk but did not identify it as a gap.
"When I first saw the gap structure, it just popped out like that," Debes says. "The fact that we see the gap at every wavelength tells you that it's a structural feature rather than an instrumental artifact or a feature of how the dust scatters light.
The team plans to use ALMA and NASA's upcoming James Webb Space Telescope, an infrared observatory set to launch in 2018, to study the system in more detail.
The team's paper will appear online on June 14 in The Astrophysical Journal.
For images, illustrations, and more information about TW Hydrae, visit:
For more information about the Hubble Space Telescope, visit:
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy Inc., in Washington.
Scientists reach back in time to discover some of the most power-packed galaxies
28.02.2017 | Clemson University
From rocks in Colorado, evidence of a 'chaotic solar system'
23.02.2017 | University of Wisconsin-Madison
On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded after a glide flight with an Airbus A320 in ditching on the Hudson River. All 155 people on board were saved.
On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded...
In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport
Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...
The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.
The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...
Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...
13.02.2017 | Event News
10.02.2017 | Event News
09.02.2017 | Event News
28.02.2017 | Physics and Astronomy
28.02.2017 | Materials Sciences
28.02.2017 | Health and Medicine